It has been determined that pipelines carrying fluids such as natural gas, liquid gas, or liquid-gas phases are subject to splitting or cracking as a result of both external and internal events that locally stress the wall of the pipe beyond the stress level sustainable by the material of the pipe. If the split or crack is of any significant length, it will not merely remain static but it will propagate longitudinally in the pipeline at high velocity in both directions away from the initial split. For certain temperature conditions, the crack may propagate along the pipeline made of material such as mild steel, as a ductile crack opening the pipe up longitudinally generally along a line at the top of the pipeline while under other temperature conditions, the crack may propagate as a brittle crack as it travels along the pipeline. In contrast to the ductile crack propagation wherein the pipe actually opens up, the brittle crack appears simply as a crack or a network of cracks without significant pipe distortion. The velocity of a ductile crack may vary from about 200 to about 1,000 feet per second while the brittle crack will propagate at greater speeds, e.g. of the order of 1,300 to 3,000 feet per second. In both cases, the speed of propagation unless such propagation is in someway inhibited, the speed of propagation is greater than or the same as the velocity of the critical driving pressure wave of the gaseous or volatile contents of the pipeline, so that in the vicinity of the tip or head of the crack, the internal pressure is sufficient to continue crack propagation until in some manner the fracture is arrested. While the problem of pipeline fracture propagation is not as significant in smaller diameter pipelines carrying lower pressure gas under moderate climatic conditions, the problem becomes a serious one in connection with larger diameter pipelines carrying high pressure gaseous fluid rich in hydrocarbons in extreme climatic conditions an environmental factors which all may be present in, for example, piping natural gas and hydrocarbons from the Arctic.
Regardless of the care taken for obtaining the desired pipe specifications including built-in safety margins to handle the anticipated operating conditions of the pipeline, pipeline cracking, even possibly brittle cracking, cannot be ruled out. Moreover, since pipelines may be employed for transporting natural gas and the like from remote areas such as the Arctic, the damage of fracture propagation over long lengths of pipeline in remote and inaccessible areas is apparent. It is therefore necessary to provide for the arresting of any propagating crack before such crack can propagate over any extensive distance and to achieve such crack arrest in an economical way.